1. Field of the Invention
The present invention relates to a power transfer device for four-wheel drive vehicles, and more particularly to a manually operated change-speed mechanism in the power transfer device.
2. Description of the Prior Art
In Japanese Utility Model Early Publication No. 61-153628, there has been proposed a manually operated ancillary change-speed mechanism in a power transfer device for four-wheel drive vehicles. The change-speed mechanism comprises input and output shafts coaxially arranged to each other for relative rotation, a planetary gear unit having an input element mounted on the input shaft for rotation therewith and an output element connectable to the output shaft, and a clutch sleeve axially slidably mounted on the output shaft for rotation therewith and shiftable between a first position in which it is engaged with the output element of the gear unit to provide a low speed drive power train between the input and output shafts and a second position in which it is engaged with the input shaft to provide a high speed drive power train between the input and output shafts. The clutch sleeve has an internal spline in continual engagement with an externally splined portion of the output shaft and engageable with an externally splined portion of the input shaft and has an external spline engageable with an internal spline of the output element of the gear unit. When positioned in the first position, the clutch sleeve is maintained in meshing engagement with the internal spline of the output element at its external spline to provide the low speed drive power train between the input and output shafts. When shifted to the second position, the clutch sleeve is brought into meshing engagement with the externally splined portion of the input shaft at its internal spline to establish the high speed drive power train between the input and output shafts.
In such a change-speed mechanism as described above, smooth meshing engagment with the externally splined portion of the input shaft may not be effected in shifting operation of the clutch sleeve to the second position from the first position. For this reason, unpleasant gear noises will occur if the clutch sleeve is shifted to the second position during acceleration or high speed travel of the vehicle. To eliminate the occurrence of such unpleasant gear noises, the clutch sleeve has to be shifted to the second position only when the vehicle is stopping or traveling at a low speed.